net: l3mdev: prefer VRF master for source address selection

When selecting an address in context of a VRF, the vrf master should be
preferred for address selection.  If it isn't, the user has a hard time
getting the system to select to their preference - the code will pick
the address off the first in-VRF interface it can find, which on a
router could well be a non-routable address.
Signed-off-by: David Lamparter <equinox@diac24.net>
Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
[dsa: Fixed comment style and removed extra blank link ]
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/devinet.c

@@ -1217,6 +1217,23 @@ __be32 inet_select_addr(const struct net_device *dev, __be32 dst, int scope)
 no_in_dev:
 	master_idx = l3mdev_master_ifindex_rcu(dev);
 
+	/* For VRFs, the VRF device takes the place of the loopback device,
+	 * with addresses on it being preferred.  Note in such cases the
+	 * loopback device will be among the devices that fail the master_idx
+	 * equality check in the loop below.
+	 */
+	if (master_idx &&
+	    (dev = dev_get_by_index_rcu(net, master_idx)) &&
+	    (in_dev = __in_dev_get_rcu(dev))) {
+		for_primary_ifa(in_dev) {
+			if (ifa->ifa_scope != RT_SCOPE_LINK &&
+			    ifa->ifa_scope <= scope) {
+				addr = ifa->ifa_local;
+				goto out_unlock;
+			}
+		} endfor_ifa(in_dev);
+	}
+
 	/* Not loopback addresses on loopback should be preferred
 	   in this case. It is important that lo is the first interface
 	   in dev_base list.